Reboilers in thermally linked columns of air separation plants are generally of the thermosiphon type. In many cases, the fluids exchanging heat are relatively pure nitrogen on the high temperature side and pure or impure oxygen on the low temperature side. The nitrogen condenses in downflow and serves as the reflux for the high pressure column, while the oxygen boils in upflow and serves as the boil-up for the low pressure column. The pressure in the high pressure column drives the flow of the nitrogen through the condensing side of the heat exchanger and the condensed nitrogen is then allowed to build static head equivalent to the pressure drop for it to flow back into the high pressure column. The flow on the oxygen side on the other hand is driven by the density difference between the outside of the exchanger, which is essentially all liquid, and the inside of the exchanger, which is part vapor and part liquid. The heat exchanger is usually completely or partially submerged in the oxygen it boils. The resulting cooling curves are not parallel and this feature limits the approach temperatures of the two streams. For a given pressure in the low pressure column, this increases the pressure at which the high pressure column has to operate, and thereby the power consumption of the main air compressor. Any innovation that allows the two stream temperatures to approach more closely in a parallel fashion would be beneficial in terms of the overall thermodynamic efficiency of the plant. It should be pointed out that although the above problem has been described in terms of the main reboiler/condenser of an air separation column the nonparallel cooling curves can occur in other reboiler/condensers in an air separation plant or any thermosiphons used in the heat exchanger industry. There would be potential improvements in thermodynamic efficiencies in all such situations by rendering the cooling curves parallel by some engineering modification.
The drive towards more energy efficient air separation plants, especially of large size, has produced many advances in the traditional areas such as the distillation columns, compressors, pumps and expanders. Heat exchangers, specifically the reboiler/condensers, are also a potential area for significant gains. Just as the falling film evaporators commonly used in the food industry have demonstrated, the advantages of downflow boiling can also be of value to the cryogenic air separation industry. Several patents make references to this concept and the following discussion will highlight their key features and the shortcomings that the current invention disclosure attempts to remedy.
EP 0 303 492 A2 discloses a method of enhancing heat transfer coefficients for boiling by spraying the surface with a thermally conductive coating consisting of metallic and plastic particles. The reference cites experimental results that show the advantages of the sprayed surface over the unsprayed surface in pool boiling and of the sprayed surface over both of the above when boiling is in downflow. The reference makes specific references to reboiler/condensers used in air separation columns wherein the boiling is in downflow. The boiling liquid distribution is via a single stage intra-passage distribution using orifices from the top. The reference teaches that a typical exchanger has a spacing of about 100 mm with 6 mm high fins and 2.5 mm fin gap.
U.S. Pat. No. Re 33,026 teaches a downflow heat exchanger which incorporates predistribution of a boiling liquid for reboil, e.g. liquid oxygen, by holes and fine distribution by means of a packing to form a continuous running liquid film. This principle is particularly applicable to air separation plants. While predistribution is accomplished by means of orifices, fine distribution can be achieved by means of serrated hardway finning or by means of a sprayed liquid on the primary surfaces or the parting sheets. Enhancement to distribution by horizontal ribbing is mentioned.
Australian Pat. No. 28509/71 teaches a reboiler/condenser incorporating two stage or one stage distribution with restrictions, namely through orifices, that cause flashing to form vapor from the boiling liquid feed in order to get a two-phase mixture in the distribution zone.
U.S. Pat. No. 3,992,168 teaches an exchanger which is a condenser and rectifier in one core. The core taught by this patent has provisions for splitting the vapor and liquid phases in the boiling stream, such that the vapor feeds directly from the header into the finning while the liquid has to pass through perforations before it rejoins the vapor. This backup upstream of these perforations is the coarse distribution analogous to the predistribution in U.S. Pat. No. Re 33,026. Another feature mentioned in the patent is decreasing fin density along the boiling side to reduce the pressure drop thereby accommodating the increasing vapor content.
U.S. Pat No. 4,646,822 discloses a mixing device that is used to distribute two-phase mixtures uniformly into the passages of a heat exchanger. The mixing device can be applied to both the hot and cold streams when they each consist of two phases. The approach is to introduce one phase, preferably the vapor, at one end of the core from a header into each passage and the other phase, preferably the liquid, from a header via slots with and without orifices into each passage where the latter phase mixes with the former. The pressure drop in the fins downstream of the mixing device is stated to ensure that the fluid is distributed uniformly. Several embodiments are shown which are different in mechanical detail but not in the purpose. The hot and cold streams are shown to be flowing in countercurrent fashion. The orientation of the core is not stated clearly to ascertain if the boiling occurs in upflow or downflow.
This patent is relevant only when it is viewed in the restricted case of downflow boiling wherein the phase distributed through the header via slots is the liquid phase.
A shortcoming that is common to all the above references is that they attempt to distribute the boiling fluid only at the inlet to the core but do not provide any means to correct a boiling liquid's natural tendency to maldistribute and form dry patches as it evaporates in downflow. It is well known that dry patches are detrimental to heat transfer and good wetting of all the boiling surfaces has to be maintained especially for near complete evaporation.